I've never used a sig 30w mini, but from my Electrical Engineering knowledge I can explain a little about these modes. The PWM mode is used as a regulated power mode, and the DC-DC is used for mechanical
mod mode. I don't have much knowledge/experience with these in terms of
vaping, but from what I can understand the DC-DC is simply a 'static' voltage, meaning it wont change, which is assuming why you're getting 4.9v, but with the PWM, you can alter the voltage, so it is 'dynamic' (Because PWM is pulse width modulation, bigger gaps in the pulses meaning lower voltage etc)
Edit: I did a quick bit of research and I found this:
Mechanical mode bypasses the chip-set and allows to sub-ohm lower than the chip allows
I also found this
thread on reddit which has a big post regarding the different modes
REVIEW OF THE SIGELEI MINI 30W!!! : electronic_cigarette